Video decoding method and apparatus using the same

ABSTRACT

Disclosed is a video decoding method that decodes a bitstream, the method including receiving a picture parameter set (PPS) comprising at least one of first information indicating whether the same reference picture list is applied to slices comprised in a picture and second information indicating whether additional information on modification of the reference picture list is present, and deriving a construction of the reference picture list based on the PPS. Accordingly, there are provided a method and an apparatus for signaling by a picture whether the construction of the reference picture list is modified when constructing the reference picture list.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/845,770, filed Dec. 18, 2017, which is a continuation of U.S.application Ser. No. 15/418,888, filed Jan. 30, 2017, which is acontinuation of U.S. application Ser. No. 14/352,586, filed Apr. 17,2014, assigned U.S. Pat. No. 9,560,369, which is a U.S. National PhaseApplication of International Application No PCT/KR2013/008672, filed onSep. 27, 2013, which claims the benefit of U.S. Provisional ApplicationNo. 61/706,783, filed on Sep. 28, 2012, the entire contents of which arehereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a video compression technique, and moreparticularly, to a method and an apparatus for signaling referencepicture list information.

BACKGROUND ART

Recently, demands for high-resolution and high-quality pictures haveincreased in various fields of applications. As pictures have higherresolution and higher quality, the amount of information on the picturesalso increases.

With a growing amount of information, multi-functional devices andnetworks with various environments are introduced. Accordingly, the samecontent may be utilized with different levels of quality.

Specifically, as terminals are able to support diverse qualities ofpictures and various network environments are established, a picturewith general quality is enabled in one environment while ahigher-quality picture may be available in another environment. Forexample, a user may enjoy video content purchased through a portableterminal on a large-screen display with higher resolution at home.

In recent years, as high definition (HD) broadcast services areavailable, a large number of users are getting used to high-resolutionand high-quality videos and service providers and service users also payattention to ultrahigh-definition (UHD) services having a resolutionfour times higher than HDTV.

Thus, there is a need to provide scalability to video quality, forexample, the image quality, resolution, size and frame rate of a video,based on high-efficiency encoding and decoding methods on ahigh-capacity video so as to offer varied qualities of video services indifferent environments for users' demands.

DISCLOSURE Tehnical Problem

An aspect of the present invention is to provide a method and anapparatus for describing reference picture list information in abitstream.

Another aspect of the present invention is to provide a method and anapparatus for signaling by a picture whether modification ofconstruction of a reference picture list occurs when constructing thereference picture list.

Still another aspect of the present invention is to provide a method ofmodifying a construction of a reference picture list corresponding tocharacteristics of pictures when the reference picture list isconstructed, and an apparatus using the same.

Technical Solution

An embodiment of the present invention provides a video decoding methodthat decodes a bitstream, the method including receiving a pictureparameter set (PPS) including at least one of first informationindicating whether the same reference picture list is applied to slicesincluded in a picture and second information indicating whetheradditional information on modification of the reference picture list ispresent; and deriving a construction of the reference picture list basedon the PPS.

The second information may be included in the PPS.

The first information may be included in a video usability information(VUI) parameter.

The deriving of the construction of the reference picture list mayderive the construction of the reference picture list once for onepicture when the first information is 1.

The second information may be received when the first information is 1.

The deriving of the construction of the reference picture list mayderive the construction of the reference picture list by each sliceincluded in the picture when the first information is 0.

Another aspect of the present invention is to provide a video decodingapparatus that decodes a bitstream, the apparatus including a parsingmodule to parse a PPS including at least one of first informationindicating whether the same reference picture list is applied to slicesincluded in a picture and second information indicating whetheradditional information on modification of the reference picture list ispresent; and a prediction module to derive a construction of thereference picture list based on the first information and the secondinformation included in the PPS.

Advantageous Effects

An embodiment of the present invention provides a method and anapparatus for describing reference picture list information in abitstream.

Another embodiment of the present invention provides a method and anapparatus for signaling by a picture whether modification ofconstruction of a reference picture list occurs when constructing thereference picture list.

Still another embodiment of the present invention provides a method ofmodifying a construction of a reference picture list corresponding tocharacteristics of pictures when the reference picture list isconstructed, and an apparatus using the same.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically illustrating a video encodingapparatus according to an exemplary embodiment of the present invention.

FIG. 2 is a block diagram schematically illustrating a video decodingapparatus according to an exemplary embodiment of the present invention.

FIG. 3 schematically illustrates an available candidate block when interprediction is performed on a current block.

FIG. 4 is a flowchart schematically illustrating a method ofconstructing a reference picture list based on a reference picture set.

FIG. 5 is a block diagram schematically illustrating an apparatus forinitializing a reference picture list.

FIG. 6 illustrates a dynamic change of a reference picture list in acoded picture.

FIG. 7 illustrates a dynamic change of a reference picture listaccording to the present invention.

FIG. 8 is a flowchart illustrating a video decoding method according toan exemplary embodiment of the present invention.

MODE FOR INVENTION

The present invention may be changed and modified variously and beillustrated with reference to different exemplary embodiments, some ofwhich will be described in detail and shown in the drawings. However,these embodiments are not intended for limiting the invention. Theterminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting the technical ideaof the invention. As used herein, the singular forms “a,” “an” and “the”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. It will be further understood that theterms “include” and/or “have,” when used in this specification, specifythe presence of stated features, integers, steps, operations, elements,components, and/or combinations thereof, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or combinations thereof.

Although elements illustrated in the drawings are independently shownfor convenience of description of different distinctive functions in thevideo encoding apparatus/decoding apparatus, such a configuration doesnot indicate that each element is constructed by a separate hardwareconstituent or software constituent. That is, at least two elements maybe combined into a single element, or a single element may be dividedinto a plurality of elements to perform functions. It is to be notedthat embodiments in which some elements are integrated into one combinedelement and/or an element is divided into multiple separate elements areincluded in the scope of the present invention without departing fromthe essence of the present invention.

Hereinafter, exemplary embodiments of the invention will be described indetail with reference to the accompanying drawings. Like referencenumerals in the drawings refer to like elements throughout, andredundant descriptions of like elements will be omitted herein.

FIG. 1 is a block diagram schematically illustrating a video encodingapparatus according to an exemplary embodiment of the present invention.A scalable video encoding/decoding method or apparatus may be realizedby extension of a general video encoding/decoding method or apparatusthat does not provide scalability, and FIG. 1 illustrates an example ofa video encoding apparatus as a base of a scalable video encodingapparatus.

Referring to FIG. 1, the video encoding apparatus 100 includes a picturepartition module 105, a prediction module 110, a transform module 115, aquantization module 120, a rearrangement module 125, an entropy encodingmodule 130, a dequantization module 135, an inverse transform module140, a filter 145 and a memory 150.

The picture partition module 105 may divide an input picture into atleast one block as a process unit. Here, the block as the process unitmay be a prediction unit (PU), a transform unit (TU) or a coding unit(CU).

Process unit blocks divided by the picture partition module 105 may havea quadtree structure.

The prediction module 110 may include an inter prediction module toperform inter prediction and an intra prediction module to perform intraprediction, which will be described. The prediction module 110 generatesa prediction block by perconstructing prediction on the process unit ofthe picture from the partition module 105. A process unit of the picturein the prediction module 110 may be a CU, a TU or a PU. Furthermore, theprediction module 110 may determine whether prediction to be performedon the process unit is inter prediction or intra prediction, and maydetermine details (for example, prediction mode) of each predictionmethod. Here, a process unit on which prediction is performed may bedifferent from a process unit for which a prediction method and detailson the prediction methods are determined. For example, a predictionmethod and a prediction mode may be determined for each PU, whileprediction may be performed on each TU.

In inter prediction, a prediction block may be generated byperconstructing prediction based on information on at least one ofprevious and/or subsequent pictures of the current picture. Furthermore,in intra prediction, a prediction block may be generated byperconstructing prediction based on information on a pixel within thecurrent picture.

A skip mode, a merge mode and a motion vector prediction (MVP) may beused as an inter prediction method. In inter prediction, a referencepicture may be selected for a PU, and a reference block corresponding tothe PU may be selected. The reference block may be selected in an interpixel unit. Subsequently, a prediction block that has a minimum residualsignal with respect to the current PU and has a minimum-size motionvector is generated.

The prediction block may be generated in an integer sample unit or in apixel unit smaller than an integer pixel, such as a ½ pixel unit and a ¼pixel unit. Here, the motion vector may be represented in a unit smallerthan an integer pixel.

Information on the reference pixel selected in inter prediction, such asan index, a motion vector (e.g., a motion vector predictor) and aresidual signal of the reference picture, is subjected to entropyencoding and transferred to a decoding apparatus. In the skip mode,since the prediction block may be a reconstructed block regardless of aresidual block, the residual block may not be generated, transformed,quantized and transferred.

In intra prediction, a prediction mode is determined by a PU, andprediction may be performed by a PU. Alternatively, a prediction modemay be determined by a PU, and intra prediction may be performed in aTU.

An intra prediction mode may have 33 directional prediction modes andtwo or more non-directional modes. The non-directional modes can includea DC prediction mode and a planar mode.

In intra prediction, the prediction block may be generated afterapplying a filter to a reference sample. Here, whether or not to applythe filter to the reference sample may be determined on an intraprediction mode and/or size of a current block.

A residual value (or a residual block or a residual signal) between thegenerated prediction block and an original block is input to thetransform module 115. Also, information on a prediction mode andinformation on a motion vector used for the prediction, along with theresidual value, are encoded by the entropy encoding module 130 andtransferred to the decoding apparatus.

The transform module 115 transforms the residual block by the TU andgenerates a transform coefficient.

A transform block is a rectangular block of samples, which the sametransformation is applied to. The transform block may be a TU and have aquadtree structure.

The transform module 115 may perform transformation based on aprediction mode applied to the residual block and a size of the block.

For example, when intra prediction is applied to the residual block andthe block has a 4×4 residual array, the transform module 115 maytransform the residual block using discrete cosine transform (DCT).Otherwise, the transform module 115 may transform the residual blockusing discrete sine transform (DST).

The transform module 115 may generate a transform block of transformcoefficients by transformation.

The quantization module 120 may generate quantized transformcoefficients by quantizing residual values transformed by the transformmodule 115, that is, the transform coefficients. The coefficientsderived by the quantization module 120 are provided to thedequantization module 135 and the rearrangement module 125.

The rearrangement module 125 rearranges the quantized transformcoefficients provided by the quantization module 120. Rearranging thequantized transform coefficients may enhance encoding efficiency in theentropy encoding module 130.

The rearrangement module 125 may rearrange a two-dimensional (2D) blockof the quantized transform coefficients into a one-dimensional (1D)vector using coefficient scanning.

The entropy encoding module 130 may perform entropy encoding on symbolsaccording to probability distribution based on the quantized transformcoefficients rearranged by the rearrangement module 125 or encodingparameter values derived during the encoding process to outputbitstreams. Entropy encoding is a method of receiving symbols havingdifferent values and representing the symbols as a decodable binarysequence or string while removing statistical redundancy.

Here, a symbol means a syntax element as an encoding/decoding target, acoding parameter, a value of a residual signal, or the like. A codingparameter, which is a parameter necessary for encoding and decoding, mayinclude information encoded by the encoding apparatus and transferred tothe decoding apparatus, such as a syntax element, and informationderived during a encoding or decoding process and means informationnecessary for encoding and decoding a picture. The coding parameter mayinclude, for example, values or statistics of an intra/inter predictionmode, a movement/motion vector, a reference picture index, an encodingblock pattern, presence and absence of a residual signal, a quantizedtransform parameter, a block size and block partition information. Aresidual signal may denote a difference between an original signal and aprediction signal, a signal obtained by transconstructing the differencebetween the original signal and the prediction signal, or a signalobtained by transconstructing and quantizing the difference between theoriginal signal and the prediction signal. The residual signal may bereferred to as a residual block in a block unit.

When entropy encoding is applied, symbols are represented by allocatinga small number of bits to symbols having a high probability andallocating a large number of bits to symbols having a low probability,thereby reducing a size of bit strings for symbols to be encoded.Therefore, entropy encoding may enhance compression performance of videoencoding.

Encoding methods, such as exponential Golomb, context-adaptive variablelength coding (CAVLC) and context-adaptive binary arithmetic coding(CABAC), may be used for entropy encoding. For example, the entropyencoding module 130 may store a table used for perconstructing entropyencoding, such as a variable length coding/code (VLC) table, and theentropy encoding module 130 may perform entropy encoding using thestored VLC table. In addition, the entropy encoding module 130 mayderive a binarization method for a target symbol and a probability modelfor a target symbol/bin and perform entropy encoding using the derivedbinarization method or probability model.

Here, binarization means representing values of symbols as a binsequence/string. A bin means each bin value (0 or 1) when a symbol isrepresented as a bin sequence/string through binarization.

A probability model means a predicted probability of a symbol/bin as anencoding/decoding target that may be derived through contextinformation/context model. Context information/context model isinformation for determining a probability of a symbol/bin as anencoding/decoding target.

In more detail, CABAC as an entropy encoding method transforms a symbolthat is not binarized into a bin by binarization, determines a contextmodel using encoding information on a neighboring block and a block tobe encoded or information on a symbol/bin encoded in a previous stage,and predicts a probability of a bin according to the determined contextmodel to perform arithmetic encoding of the bin, thereby generating abitstream. Here, CABAC may determine the context model, and then updatethe context model using information on an encoded symbol/bin for acontext model for a next symbol/bin.

Furthermore, the entropy coding module 130 may apply a change to areceived parameter set or syntax as necessary.

The dequantization module 135 performs dequantization on the values(transform coefficients) quantized by the quantization module 120, andthe inverse transform module 140 performs inverse transform on thevalues dequantized by the dequantization module 135.

The residual values generated via the dequantization module 135 and theinverse transform module 140 are merged with the prediction blockpredicted by the prediction module 110 to generate a reconstructedblock.

FIG. 1 illustrates that the reconstructed block is generated by mergingthe residual block with the prediction block through an adder. Here, theadder may be regarded as a separate module for generating thereconstructed block (reconstructed block generation module).

The filter 145 may apply a deblocking filter, an adaptive loop filter(ALF), and a sample adaptive offset (SAO) to a reconstructed picture.

The deblocking filter may remove block distortion generated onboundaries between blocks in the reconstructed picture. The ALF mayperform filtering based on a value obtained by comparing thereconstructed picture with blocks filtered by the deblocking filter withthe original picture. The ALF may be employed only for high efficiency.The SAO reconstructs an offset difference between the residual blockwhich has been subjected to the deblocking filter and the originalpicture by a pixel unit, in which a band offset or an edge offset isused.

Meanwhile, the filter 145 may not apply filtering to a reconstructedblock used in inter prediction.

The memory 150 may store the reconstructed block or picture derivedthrough the filter 145. The reconstructed block or picture stored in thememory 150 may be provided to the prediction module 110 perconstructinginter prediction.

FIG. 2 is a block diagram schematically showing a video decodingapparatus according to an exemplary embodiment of the present invention.As described above in FIG. 1, a scalable video encoding/decoding methodor apparatus may be realized by extension of a general videoencoding/decoding method or apparatus that does not provide scalability,and FIG. 2 illustrates an example of a video decoding apparatus as abase of a scalable video decoding apparatus.

Referring to FIG. 2, the video decoding apparatus 200 may include anentropy decoding module 210, a rearrangement module 215, andequantization module 220, an inverse transform module 225, a predictionmodule 230, a filter 235, and a memory 240.

When a video bitstream is input from the video encoding apparatus, theinput bitstream may be decoded according to an inverse procedure bywhich the video encoding apparatus processes video information.

The entropy decoding module 210 performs entropy decoding on an inputbitstream according to probability distribution to generate symbolsincluding quantized coefficient type of symbols. Entropy decoding is amethod of receiving a binary sequence or string and generating eachsymbol. Entropy decoding is similar to entropy encoding described above.

For example, if the video encoding apparatus uses variable length coding(VLC), such as CAVLC, to perform entropy encoding, the entropy decodingmodule 210 may perform entropy decoding by implementing the same VLCtable as used in the encoding apparatus. Furthermore, if the videoencoding apparatus uses CABAC to perform entropy ending, the entropydecoding module 210 may also perform entropy decoding using CABAC.

In more detail, CABAC as an entropy decoding method may receive a bincorresponding to each syntax element in the bitstream, determines acontext model using information on a syntax element to be decoded anddecoding information on a neighboring block and a block to be decoded orinformation on a symbol/bin decoded in a previous stage, and predict aprobability of a bin according to the determined context model toperform arithmetic decoding of the bin, thereby generating a symbolcorresponding to a value of each syntax element. Here, CABAC maydetermine the context model, and then update the context model usinginformation on a decoded symbol/bin for a context model for a nextsymbol/bin.

When entropy decoding is applied, symbols are represented by allocatinga small number of bits to symbols having a high probability andallocating a large number of bits to symbols having a low probability,thereby reducing a size of bit strings for each symbol. Therefore,entropy decoding may enhance compression performance of video decoding.

Information for generating a prediction block, among pieces ofinformation decoded by the entropy decoding module 210, may be providedto the prediction module 230. Residual values entropy-decoded by theentropy decoding module 210, that is, quantized transform coefficients,may be input to the rearrangement module 215.

The rearrangement module 215 may rearrange the information on thebitstream entropy-decoded by the entropy decoding module 210, that is,the quantized transform coefficients, based on a rearrangement methodused in the encoding apparatus.

The rearrangement module 215 may reconstruct and rearrange coefficientsexpressed in a 1D vector form into coefficients in a 2D block. Therearrangement module 215 may generate the coefficients in the 2D block(quantized transform coefficients) by scanning the coefficients based ona prediction mode and a size of a transform block applied to the currentblock (transform block).

The dequantization module 220 may perform dequantization based on aquantization parameter provided from the encoding apparatus and therearranged coefficients of the block. The inverse transform module 225may perform inverse DCT and/or inverse DST on a result of quantizationperformed by the video encoding apparatus, having been subjected to DCTand DST performed by the transform module of the encoding apparatus.

Inverse transform may be performed on the basis of a transfer unit or apartition unit of a picture determined by the video encoding apparatus.The transform module of the video encoding apparatus may selectivelyperform DCT and/or DST depending on a plurality of information elements,such as a prediction method, a size of the current block and aprediction direction, and the inverse transform module 225 of the videodecoding apparatus may perform inverse transform on the basis ofinformation on the transform performed by the transform module of thevideo encoding apparatus.

The prediction module 230 may generate a prediction block based oninformation about generation of the prediction block provided from theentropy decoding module 210 and information on a previously decodedblock and/or picture provided by the memory 240.

If a prediction mode for a current PU is an intra prediction mode, intraprediction may be performed based on information on a pixel in a currentpicture to generate the prediction block.

If a prediction mode for the current PU is an inter prediction mode,inter prediction for the current PU may be performed based oninformation included in at least one of previous and subsequent picturesof the current picture. Here, motion information necessary for the interprediction for the current PU provided by the video encoding apparatus,for example, information on a motion vector and an index of a referencepicture, may be derived by checking a skip flag and a merge flagreceived from the encoding apparatus.

A reconstructed block may be generated using the prediction blockgenerated by the prediction module 230 and the residual block providedby the inverse transform module 225. FIG. 2 illustrates that thereconstructed block is generated by merging the prediction block withthe residual block by the adder. Here, the adder may be regarded as aseparate module for generating the reconstructed block (reconstructedblock generation module).

When the skip mode is used, the residual block is not transmitted andthe prediction block is the reconstructed block.

The reconstructed block and/or picture may be provided to the filter235. The filter 235 may apply deblocking filtering, SAO and/or AFL tothe reconstructed block and/or picture.

The memory 240 may store the reconstructed picture or block to be usedas a reference picture or a reference block and supply the reconstructedpicture to an output unit. Components directly related to video decodingamong the entropy decoding module 210, the rearrangement module 215, thedequantization module 220, the inverse transform module 225, theprediction module 230, the filter 235 and the memory 240 of the decodingapparatus 200, for example, the entropy decoding module 210, therearrangement module 215, the dequantization module 220, the inversetransform module 225, the prediction module 230 and the filter 235 maybe defined as a decoder or a decoding unit, separately from the othercomponents.

Further, the decoding apparatus 200 may further include a parsing module(not shown) to parse information about an encoded video included in thebitstream. The parsing module may include the entropy decoding module210 or be included in the entropy decoding module 210. The parsingmodule may be provided as one component of the decoding unit.

FIG. 3 schematically illustrates an available candidate block when interprediction is performed on a current block according to an exemplaryembodiment.

The prediction modules of the encoding apparatus and the decodingapparatus may use a block at a preset location neighboring to a currentblock 300 as a candidate block. For example, in FIG. 3, two blocks A₀310 and A₁ 320 located at a bottom left side of the current block andthree blocks B₀ 330, B₁ 340 and B₂ 350 located at top right and top leftsides of the current block may be selected as spatial candidate blocks.In addition to the spatial neighboring blocks, a Col block 360 may beused as a temporal candidate block.

Regarding reference pictures used for inter prediction, a referencepicture for the current block may be derived from a reference picturefor a neighboring block or indicated by information received from theencoding apparatus. In the skip mode or merge mode, the predictionmodule of the decoding apparatus may use the reference picture for theneighboring block as the reference picture for the current picture. Whenthe MVP is applied, the prediction module of the decoding apparatus mayreceive information indicating the reference picture for the currentblock from the encoding apparatus.

Pictures encoded/decoded prior to a current picture may be stored in amemory, for example, a decoded picture buffer (DPB), and be used forprediction of the current block or current picture. Pictures used forinter prediction of the current block may be derived as a referencepicture list. Here, a reference picture used for inter prediction of thecurrent block among the reference pictures included in the referencepicture list may be indicated by a reference picture index. That is, thereference picture index may refer to an index indicating the referencepicture used for inter prediction of the current block among thereference pictures constructing the reference picture list.

An I slice is a slice decoded by intra prediction. A P slice is a slicedecoded by intra prediction or inter prediction using at most one motionvector and one reference picture. A B slice is a slice decoded by intraprediction or inter prediction using at most two motion vectors and tworeference pictures. Here, the reference pictures may include short-termreference pictures (STRPs) and long-term reference pictures (LTRPs).

Here, the STRPs and the LTRPs may be reconstructed pictures stored inthe DPB. The STRPs may be marked as “used for short-term reference” or“used for reference.” The LTRPs may be marked as “used for long-termreference” or “used for reference.” For instance, a difference inpicture order count (POC) between the STRPs and the LTRP may have avalue ranging from 1to 2²⁴-1. Here, POC may refer to display order ofpictures.

Reference picture list 0 (L0) is a reference picture list used for interprediction of a P slice or B slice. Reference picture list 1 (L1) may beused for inter prediction of a B slice. Thus, uni-directional predictionbased on L0 may be performed for inter prediction of a block of a Pslice, while bi-prediction based on L0 and L1 may be performed for interprediction of a block of a B slice.

The encoding apparatus and/or the decoding apparatus may construct areference picture list when encoding and/or decoding is performed on a Pslice and a B slice through inter prediction. Here, a reference pictureused for inter prediction may be designated through a reference pictureindex. As described above, a reference picture index may refer to anindex indicating a reference picture in a reference picture list usedfor inter prediction.

The reference picture list may be constructed based on a referencepicture set determined or generated by the encoding apparatus and thedecoding apparatus. Reference pictures constructing the referencepicture list may be stored in the memory, for example, the DPB. Thepictures, which are encoded/decoded prior to the current picture, storedin the memory may be managed by the encoding apparatus and the decodingapparatus.

A sliding window method, which is used to manage the reference pictures,may simply manage the reference pictures by removing the referencepictures after a predetermined time since the reference pictures arestored in the memory and but have several problems. For example, areference picture which is not needed any more may not be immediatelyremoved, thus reducing efficiency. Moreover, the stored referencepictures are removed from the memory after the predetermined time,making it difficult to manage LTRPs.

In view of the problems of the slide window method, a memory managementcommand operation (MMCO) method in which instruction information onmanagement of the reference pictures is signalled directly from theencoding apparatus may be used. Particularly, in the MMCO method, acommand to allocate a picture as an LTRP, a command to change an LTRP toan STRP and a command to mark an LTRP as “unused for reference” may bedefined with respect to LTRP management.

However, even in use of the MMCO method, picture loss may occur during asignaling process. If a lost picture includes an MMCO command, lost MMCOinformation may not be reconstructed, so that the memory or DPB may notbe maintained in a proper state that currently needed pictures aremanaged. Thus, inter prediction may be performed inaccurately.

To solve the foregoing problems, information related to a referencepicture set needed for decoding a slice and/or picture may betransmitted in a sequence parameter set (SPS), a picture parameter set(PPS) and/or a slice header. Here, a reference picture list may beconstructed based on the reference picture set.

The reference picture set may include reference pictures used forreference for the current picture/slice or a future picture/slice. Thereference pictures used for decoding the slice and/or picture mayinclude STRPs and LTRPs. Further, the STRPs may include forward STRPshaving a lower POC than the current picture and backward STRPs having ahigher POC than the current picture. Here, the reference picture set maybe determined or generated with respect to each of the forward STRPs,the backward STRPs and the LTRPs.

Hereinafter, for convenience of description, a reference picture set offorward STRPs is defined as a forward STRP set, a reference picture setof backward STRPs as a backward STRP set, and a reference picture set ofLTRPs as an LTRP set. For example, the forward STRP set may berepresented as RefPicSetStCurrBefore, the backward STRP set asRefPicSetStCurrAfter, and the LTRP set as RefPicSetLtCurr.

FIG. 4 is a flowchart schematically illustrating a method ofconstructing a reference picture list based on a reference picture setaccording to an exemplary embodiment.

The embodiment shown in FIG. 4 will be illustrated with reference tooperations of the decoding apparatus for convenience of description. Aprocess of generating a reference picture list, which will be described,may be considered as a reference picture list initialization process.

Referring to FIG. 4, the decoding apparatus may construct a referencepicture set based on information on the reference picture settransmitted from the encoding apparatus (S410). For example, thereference picture set may be constructed for each picture to which interprediction is applied.

Here, the reference picture set may include a forward STRP set, abackward STRP set and an LTRP set. Reference pictures included in eachreference picture set may be specified by POCs. POC may refer to displayorder of pictures.

POCs of reference pictures included in the forward STRP set and thebackward STRP set may be determined on relative POC. Here, informationon the relative POC may be transmitted from the encoding apparatus tothe decoding apparatus.

Relative POC may refer to a POC difference between two pictures in areference picture set. In POC order, relative POC of previous referencepictures of a current picture that is, reference pictures having smallerPOC than POC of the current picture, may correspond to a POC differencebetween a reference picture and a reference picture just before thereference picture in a reference picture set. In the POC order, relativePOC of subsequent reference pictures of the current picture, that is,reference pictures having greater POC than the POC of the currentpicture, may corresponding to a POC difference between a referencepicture and a reference picture just before the reference picture in thereference picture set.

In the forward STRP set, forward STRPs having a smaller POC value thanthe POC of the current picture may be disposed in descending order ofPOCs. That is, pictures having a smaller POC value than the POC of thecurrent picture among pictures in the DPB may be disposed in descendingorder of POCs from a start of the forward STRP set.

In the backward STRP set, backward STRPs having a greater POC value thanthe POC of the current picture may be disposed in ascending order ofPOCs. That is, pictures having a greater POC value than the POC of thecurrent picture among the pictures in the DPB may be disposed inascending order of POCs from a start of the backward STRP set.

Reference pictures included in the LTRP set may be determined based oninformation on the LTRP set transmitted from the encoding apparatus.Here, the information on the LTRP set may include information fordetermining the reference pictures included in the LTRP set and/or POCsof the reference pictures.

Referring back to FIG. 4, the decoding apparatus may generate areference picture list based on the reference picture set (S420).

When L0 is constructed, the decoding apparatus may sequentially allocatereference picture indices to the forward STRPs constructing the forwardSTRP set, the backward STRPs constructing the backward STRP set and theLTRPs constructing the LTRP set, thereby constructing the referencepicture list. That is, in L0, the forward STRPs may be allocated, thebackward STRPs may be added, and then the LTRPs may be finally added.

The forward STRPs constructing the forward STRP set may be added to L0in the same order as included in the forward STRP set. That is, theforward STRPs may be disposed in descending order of the POCs in L0, anda greater reference index value may be allocated to a picture havingsmaller POC.

The backward STRPs constructing the backward STRP set may be added to L0in the same order as included in the backward STRP set. That is, thebackward STRPs may be disposed in ascending order of the POCs in L0, anda greater reference index value may be allocated to a picture havinggreater POC.

In addition, the LTRPs constructing the LTRP set may be added to L0 inthe same order as included in the LTRP set.

For a B slice, L1 may be also generated in addition to L0. When L1 isconstructed, the decoding apparatus may sequentially allocate referencepicture indices to the backward STRPs constructing the backward STRPset, the forward STRPs constructing the forward STRP set and the LTRPsconstructing the LTRP set, thereby constructing the reference picturelist. That is, in L1, the backward STRPs may be allocated, the forwardSTRPs may be added, and then the LTRPs may be finally added.

The backward STRPs constructing the backward STRP set may be added to L1in the same order as included in the backward STRP set. That is, thebackward STRPs may be disposed in ascending order of the POCs in L1, anda greater reference index value may be allocated to a picture havinggreater POC.

The forward STRPs constructing the forward STRP set may be added to L0in the same order as included in the forward STRP set. That is, theforward STRPs may be disposed in descending order of the POCs in L1, anda greater reference index value may be allocated to a picture havingsmaller POC. In addition, the LTRPs constructing the LTRP set may beadded to L1 in the same order as included in the LTRP set.

Reference pictures added to L0 and L1 may be sequentially allocatedreference picture indices.

The decoding apparatus may use first N reference pictures, that is,reference pictures having reference picture indices from 0 to N−1 (N isa natural number), among the reference pictures included in thereference picture lists as available reference pictures. Here,information on the number N of available reference pictures may betransmitted from the encoding apparatus. In the foregoing process, thereference picture lists may be considered to be implicitly derived. Whenthe reference picture lists are implicitly derived, the encodingapparatus and the decoding apparatus may derive the reference picturelists used for inter prediction of the current picture based on the POCsof the pictures as described above.

Meanwhile, the decoding apparatus may modify the implicitly derivedreference picture lists based on information explicitly transmitted fromthe encoding apparatus. Here, the encoding apparatus may transmit bothreference picture list modification information indicating that theimplicitly derived reference picture lists are modified and entryinformation indicating a specific entry constructing the referencepicture lists. When the reference picture lists are finally genereatedby being modified based on the information explicitly transmitted fromthe encoding apparatus, the reference picture lists may be considered tobe explicitly specified.

When L0 is explicitly specified, the encoding apparatus may transmitentry information on L0. The entry information on L0 may indicate areference picture corresponding to an index on L0. When L1 is explicitlyspecified, the encoding apparatus may transmit entry information on L1.The entry information on L1 may indicate a reference picturecorresponding to an index on L1.

For example, when the reference picture lists are explicitly specifiedby the entry information, order and/or reference picture indices of theforward STRPs, the backward STRPs and the LTRPs in the reference picturelists may be different from those in the implicitly derived referencepicture lists. Furthermore, when the reference picture lists arespecified by the entry information, available reference pictures to beutilized may be different from those in the implicitly derived referencepictures lists.

When the reference picture lists are explicitly specified, the decodingapparatus may construct the same reference picture lists as constructedby the encoding apparatus based on the reference picture listmodification information and the entry information.

In the foregoing method of implicitly deriving the reference picturelists, the reference picture set and the reference picture list areillustrated only considering available pictures for convenience ofdescription, but the encoding apparatus and the decoding apparatus mayconstruct a reference picture set and/or a reference picture list inview of whether reference pictures are available or used.

FIG. 5 is a block diagram schematically illustrating an apparatus forinitializing a reference picture list according to an exemplaryembodiment.

In the embodiment shown in FIG. 5, the apparatus for initializing thereference picture list (“reference picture list initializationapparatus”) 500 may include a reference picture set construction module510 and a reference picture list generation module 520.

Referring to FIG. 5, the reference picture set construction module 510may construct a reference picture set based on input information on thereference picture set. For instance, the reference picture set may beconstructed for each picture to which inter prediction is applied.

Here, the reference picture set may include a forward STRP set, abackward STRP set and an LTRP set. Reference pictures included in eachreference picture set may be specified by POCs. POC may refer to displayorder of pictures.

Details on operations of the reference picture set construction module510 are the same as described in operation S410 of constructing thereference picture set in FIG. 4, and thus descriptions thereof areomitted herein.

Referring back to FIG. 5, the reference picture list generation module520 may generate a reference picture list based on the reference pictureset. The reference picture list generation module 520 may generatereference picture lists L0 and L1. Details on operations of thereference picture list generation module 510 are the same as describedin operation S420 of generating the reference picture list in FIG. 4,and thus descriptions thereof are omitted herein.

The reference picture list generated by the reference picture listgeneration module 520 may be stored in a DPB for prediction ortransferred to the prediction module for reference for prediction.

Although the present embodiment shows that initialization of thereference picture list may be carried out by a separation component,that is, the reference picture list initialization apparatus forconvenience of description and understanding of the invention, thepresent invention is not limited to the embodiment. For instance,initialization of the reference picture list may be carried out by thememories, for example, a DPB, of the encoding apparatus and the decodingapparatus described with reference to FIGS. 1 and 2. In this instance,the reference picture list initialization apparatus may be the DPB.Alternatively, initialization of the reference picture list may becarried out by the prediction modules of the encoding apparatus and thedecoding apparatus shown in FIGS. 1 and 2. In this case, the referencepicture list initialization apparatus may be the prediction modules. Inaddition, the reference picture list initialization apparatus may beincluded in the encoding apparatus and the decoding apparatus as aseparate component.

The same reference picture list may be used for all slices of a pictureor different reference picture lists may be used by slices. Informationindicating whether slices included in one picture use the same referencepicture list may be signalled as a flag such asrestricted_ref_pic_lists_flag, and such flag information may be includedin an SPS.

restricted_ref_pic_lists_flag may indicate whether all P slices and Bslices (if present) included in a picture use the same L0 and whetherall B slices (if present) included in the picture use the same L1.

restricted_ref_pic_lists_flag of 1 indicates that a reference pictureconstruction process is carried out once for each picture, not for eachindividual slice, and restricted_ref_pic_lists_flag of 0 indicates thatthe reference picture construction process is not carried out once for apicture.

When restricted_ref_pic_lists_flag is 1, complexity of the referencepicture construction process is improved.

When restricted_ref_pic_lists_flag is signalled as 1, flag informationsuch as lists_modification_present_flag may be additionally signalled.

lists_modification_present_flag is a flag signal indicating whetheradditional information on modification of a reference picture list ispresent. For example, lists_modification_present_flag of 1 indicatesthat the additional information on the modification of the referencepicture list is present. When restricted_ref_pic_lists_flag is 0,lists_modification_present_flag is apparently 1, and thuslists_modification_present_flag is not signalled.

Meanwhile, when restricted_ref_pic_lists_flag is included in the SPS, itis signalled whether the reference picture construction process iscarried out once for each picture, not for each individual slice acrossa bitstream. Thus, such a signalling method is slightly inflexible andhas limitations in transferring information on the reference picturelist.

Practically, for some of bitstreams including uncomplicated andmonotonous scenes, order of reference pictures in a reference picturelist may not need changing by slices. On the contrary, for some ofbitstreams including complicated scenes, order of reference pictures ina reference picture list may need changing by slices.

FIG. 6 illustrates a dynamic change of a reference picture list in acoded picture.

As shown in FIG. 6, slices included in one coded picture may use thesame reference picture list or use different reference picture lists,respectively.

All slices included in some pictures A, for example, pictures for aspecific period of time, among pictures included in a bitstream use thesame reference picture list, while slices included in pictures B foranother specific period of time use different reference picture lists,respectively. Some slices included in one picture may use the samereference picture list, while other slices may use different referencepicture lists.

In the bitstream of FIG. 6, when restricted_ref_pic_lists_flag issignalled in an SPS, restricted_ref_pic_lists_flag is 0. Thus, thedecoding apparatus cannot take advantage of possibility to apply singlereference picture list construction per picture for the period wherereference picture list within a picture is identical, for example, thepictures A.

The present invention suggest changing a signaling location ofrestricted_ref_pic_lists_flag or/and lists_modification_present_flagdescribed above to improve a feature of limiting construction of areference picture list.

Table 1 illustrates a picture parameter set (PPS) according to anexemplary embodiment of the present invention.

TABLE 1 Descriptor pic_parameter_set_rbsp( ) { ...restricted_ref_pic_lists_flag u(1) if( restricted_ref_pic_lists_flag )lists_modification_present_flag u(1) ... }

Referring to Table 1, restricted_ref_pic_lists_flag indicates whetherall P slices and B slices (if present) included in a picture use thesame L0 and whether all B slices (if present) included in the pictureuse the same L1. restricted_ref_pic_lists_flag of 1 indicates that allslices included in a picture use the same reference picture list, whilerestricted_ref_pic_lists_flag of 0 indicates that all slices included inthe picture do not use the same reference picture lists.

lists_modification_present_flag indicates whether additional informationon modification of a reference picture list is present.lists_modification_present_flag of 1 indicates that an additional syntaxelement about modification of the reference picture list is present in aslice level, for example, a slice header, whilelists_modification_present_flag of 0 indicates that an additional syntaxelement about modification of the reference picture list is not presentin the slice header.

In the present embodiment, both restricted_ ref_pic_lists_flag andlists_modification_present_flag are signalled, being included in a PPS,instead of in an SPS. In this case, whenever the reference picture listis modified in the bitstream, modification of the reference picture listmay be signalled in real time. The encoding apparatus and the decodingapparatus may easily encode and decode information on whether theconstruction of the reference picture list is applied to each individualpicture.

Tables 2 and 3 illustrates an SPS and a PPS according to an exemplaryembodiment of the present invention.

TABLE 2 Descriptor seq_parameter_set_rbsp( ) { ...lists_modification_present_flag u(1) ... }

TABLE 3 Descriptor pic_parameter_set_rbsp( ) { ...restricted_ref_pic_lists_flag u(1) ... }

Referring to Table 2, lists_modification_present_flag indicates whetheradditional information on modification of a reference picture list ispresent. lists_modification_present_flag of 1 indicates that anadditional syntax element about modification of the reference picturelist is present in a slice level, for example, a slice header, whilelists_modification_present_flag of 0 indicates that an additional syntaxelement about modification of the reference picture list is not presentin the slice header.

lists_modification_present_flag may be signalled, being included in theSPS, as show in Table 2.

Referring to Table 3, restricted_ref_pic_lists_flag indicates whetherall P slices and B slices (if present) included in a picture use thesame L0 and whether all B slices (if present) included in the pictureuse the same L1.

restricted_ref_pic_lists_flag may be signalled, being included in thePPS instead of in the SPS.

FIG. 7 illustrates a dynamic change of a reference picture listaccording to the present invention.

As shown in FIG. 7, slices included in one coded picture may use thesame reference picture list or use different reference picture lists,respectively. When a construction of the reference picture list ismodified, modified information may be signalled by each picture, therebyefficiently signalling information on the reference picture list andenhancing coding efficiency.

For PPS 0 and PPS 2 for pictures each of which includes slices using thesame reference picture list, restricted_ref_pic_lists_flag is signalledas 1, in which case lists_modification_present_flag will be additionallysignalled. That is, when new information is added to a PPS, a new PPS issignalled accordingly.

For PPS 1 and PPS 3 for pictures each of which includes slices usingdifferent reference picture lists, restricted_ref_pic_lists_flag issignalled as 0.

Meanwhile, information on whether a reference picture list is modifiedis signaled for each picture, restricted_ref_pic_lists_flag may besignalled, being included in parameter information other than a PPS. Forexample, restricted_ref_pic_lists_flag may be signaled, being includedin a video usability information (VUI) parameter.

Tables 4 and 5 illustrate a VUI parameter and a PPS according to anexemplary embodiment of the present invention.

TABLE 4 Descriptor vui_parameters( ) { ... restricted_ref_pic_lists_flagu(1) ... }

TABLE 5 Descriptor pic_parameter_set_rbsp( ) { ...lists_modification_present_flag u(1) ... }

Referring to Table 4, restricted_ref_pic_lists_flag indicates whetherall P slices and B slices (if present) included in a picture use thesame L0 and whether all B slices (if present) included in the pictureuse the same L1.

restricted_ref_pic_lists_flag may be signalled, being included in theVUI parameter instead of in the SPS.

Referring to Table 5, lists_modification_present_flag indicates whetheradditional information on modification of a reference picture list ispresent. lists_modification_present_flag of 1 indicates that anadditional syntax element about modification of the reference picturelist is present in a slice level, for example, a slice header, whilelists_modification_present_flag of 0 indicates that an additional syntaxelement about modification of the reference picture list is not presentin the slice header.

lists_modification_present_flag may be signalled separately fromrestricted_ref_pic_lists_flag, being included in the PPS, as show inTable 5.

FIG. 8 is a flowchart illustrating a video decoding method according toan exemplary embodiment of the present invention.

First, the decoding apparatus receives and derives first informationindicating whether the same reference picture list is applied to slicesincluded in a picture (S810).

The first information may be restricted_ref_pic_lists_flag, and suchflag information may be received, being included in a PPS or a VUIparameter instead of in an SPS.

When restricted_ref_pic_lists_flag is 0 (S820), which indicates that theslices in the picture does not use the same reference picture list, thedecoding apparatus constructs a reference picture list usingmodification information on the reference picture list transmitted in aslice level, that is, information on an additional reference picturelist that may be modified from the initial reference picture list(S830).

When restricted_ref_pic_lists_flag is 1 (S820), the decoding apparatusmay receive second information indicating whether additional informationon modification of the reference picture list is present.

The second information may be lists_modification_present_flag, and suchflag information may be transmitted, being included in the SPS asconventionally or in the PPS.

When lists_modification_present_flag is 1 (S840), which indicates thatthe additional information on modification of the reference picture listis present, the decoding apparatus constructs a reference picture listusing the additional information on the reference picture list (S830).

However, when lists_modification_present_flag is 0 (S840), one referencepicture list is applied to the picture, the decoding apparatus mayconstruct the initial reference picture list using information on areference picture list transmitted in an SPS level or PPS level, and theinitial reference picture list may be used as the reference picture listfor the picture (S850).

The reference picture list may be generated based on a reference pictureset, details of which are substantially the same as mentioned above withreference to FIGS. 4 and 5 and thus are not made herein to avoidredundancy.

As described above, the present invention discloses a method and anapparatus for signaling by a picture whether a construction of areference picture list is modified when constructing the referencepicture list. The present invention may change the construction of thereference picture list corresponding to characteristics of pictures whenconstructing the reference picture list.

Although methods of illustrative systems have been described with aseries of stages or blocks based on the flowcharts, the presentinvention is not limited to the foregoing sequence of the stages. Somestages may be carried out in different order from described above or atthe same time. Further, it should be noted that as the aforementionedembodiments may include various aspects of examples, combinations of theembodiments may be also understood as exemplary embodiments of thepresent invention. Thus, it will be appreciated by those skilled in theart that changes, modifications and alternatives may be made in theseexemplary embodiments without departing from the principles and spiritof be the invention, the scope of which is defined in the appendedclaims and their equivalents.

1. (canceled)
 2. A video decoding method, comprising: obtaining, by adecoding apparatus, a lists_modification_present_flag indicating whetherinformation on modification of reference picture lists is present;obtaining, by the decoding apparatus, residual signal of a currentblock; deriving, by the decoding apparatus, a reference picture listbased on a value of the lists modification present flag; deriving, bythe decoding apparatus, a prediction sample of the current block basedon the reference picture list; deriving, by the decoding apparatus, aresidual sample of the current block based on the residual signal; andgenerating, by the decoding apparatus, a reconstructed sample based onthe prediction sample and the residual sample, wherein the listsmodification present flag is included in a picture parameter set (PPS).3. The method of claim 2, further obtaining a restricted referencepicture list flag from a sequence parameter set (SPS), wherein therestricted reference picture list flag indicates whether slices thatbelong to a current picture have an identical reference picture list,wherein the lists modification present flag is comprised in the PPS whena value of the restricted reference picture list flag is
 0. 4. Themethod of claim 3, wherein the identical reference picture list isconstructed for the slices comprised in the current picture when thevalue of the restricted reference picture list flag is
 1. 5. The methodof claim 2, wherein the reference picture list is derived based on atleast one of a forward short-term reference picture (STRP) setcomprising pictures having smaller picture order count (POC) than POC ofthe current picture, a backward STRP set comprising pictures havinggreater POC than the POC of the current picture, and a long-termreference picture (LTRP) set.
 6. The method of claim 5, wherein thereference picture list includes reference picture list 0 and referencepicture list 1, wherein the reference picture list 0 is constructed bysequentially allocating reference picture indices to forward STRPsforming the forward STRP set, backward STRPs forming the backward STRPset and LTRPs forming the LTRP set; and wherein the reference picturelist 1 is constructed by sequentially allocating reference pictureindices to the backward STRPs forming the backward STRP set, the forwardSTRPs forming the forward STRP set and the LTRPs forming the LTRP setwhen the prediction target block is a B slice.
 7. The method of claim 2,wherein deriving the reference picture list comprises: deriving atemporary reference picture list based on decoded pictures; and derivingthe temporary reference picture list as the reference picture list whenthe lists modification present flag indicates that the information onmodification of the reference picture lists is not present.
 8. Themethod of claim 2, wherein deriving the reference picture listcomprises: deriving a temporary reference picture list based on decodedpictures; and deriving the reference picture list by modifying thetemporary reference picture list based on the information onmodification of the reference picture lists when the lists modificationpresent flag indicates that the information on modification of thereference picture lists is present.
 9. A video encoding method,comprising: deriving, by an encoding apparatus, a reference picture listfor inter prediction; deriving, by the encoding apparatus, a predictionsample of a current block based on the reference picture list; deriving,by the encoding apparatus, a residual sample of the current block;determining, by the encoding apparatus, whether to signal information onmodification of reference picture lists; deriving, by the encodingapparatus, a lists modification present flag indicating whether theinformation on modification of the reference picture lists is presentbased on the result of the determination; deriving, by the encodingapparatus, residual signal based on the residual sample; and encoding,by the encoding apparatus, video information including the listsmodification present flag and the residual signal, wherein the listsmodification present flag is comprised in a picture parameter set (PPS)of the video information.
 10. The method of claim 9, wherein the videoinformation including a restricted reference picture list flag in asequence parameter set (SPS) of the video information, wherein therestricted reference picture list flag indicates whether slices thatbelong to a current picture have an identical reference picture list,wherein the lists modification present flag is comprised in the PPS whena value of the restricted reference picture list flag is
 0. 11. Themethod of claim 10, wherein the identical reference picture list isconstructed for the slices comprised in the current picture when thevalue of the restricted reference picture list flag is
 1. 12. The methodof claim 9, wherein the reference picture list is derived based on atleast one of a forward short-term reference picture (STRP) setcomprising pictures having smaller picture order count (POC) than POC ofthe current picture, a backward STRP set comprising pictures havinggreater POC than the POC of the current picture, and a long-termreference picture (LTRP) set.
 13. The method of claim 12, wherein thereference picture list includes reference picture list 0 and referencepicture list 1, wherein the reference picture list 0 is constructed bysequentially allocating reference picture indices to forward STRPsforming the forward STRP set, backward STRPs forming the backward STRPset and LTRPs forming the LTRP set; and wherein the reference picturelist 1 is constructed by sequentially allocating reference pictureindices to the backward STRPs forming the backward STRP set, the forwardSTRPs forming the forward STRP set and the LTRPs forming the LTRP setwhen the prediction target block is a B slice.
 14. The method of claim9, wherein deriving the reference picture list comprises: deriving atemporary reference picture list based on decoded pictures; and derivingthe temporary reference picture list as the reference picture list whenthe lists modification present flag indicates that the information onmodification of the reference picture lists is not present.
 15. Themethod of claim 9, wherein deriving the reference picture listcomprises: deriving a temporary reference picture list based on decodedpictures; and deriving the reference picture list by modifying thetemporary reference picture list based on the information onmodification of the reference picture lists when the lists modificationpresent flag indicates that the information on modification of thereference picture lists is present.
 16. A non-transitorydecoder-readable storage medium storing a video data, the video datacomprising a decoder executable program, the decoder executable program,when executed, causing a decoder to perform the following steps:obtaining a lists modification present flag indicating whetherinformation on modification of reference picture lists is present;obtaining residual signal of a current block; deriving a referencepicture list based on a value of the lists modification present flag;deriving a prediction sample of the current block based on the referencepicture list; deriving a residual sample of the current block based onthe residual signal; and generating a reconstructed sample based on theprediction sample and the residual sample, wherein the listsmodification present flag is included in a picture parameter set (PPS)of the video data.
 17. The non-transitory decoder-readable storagemedium of claim 16, wherein video information including a restrictedreference picture list flag in a sequence parameter set (SPS) of thevideo information, wherein the restricted reference picture list flagindicates whether slices that belong to a current picture have anidentical reference picture list, wherein the lists modification presentflag is comprised in the PPS when a value of the restricted referencepicture list flag is
 0. 18. The non-transitory decoder-readable storagemedium of claim 17, wherein the identical reference picture list isconstructed for the slices comprised in the current picture when thevalue of the restricted reference picture list flag is
 1. 19. Thenon-transitory decoder-readable storage medium of claim 16, wherein thereference picture list is derived based on at least one of a forwardshort-term reference picture (STRP) set comprising pictures havingsmaller picture order count (POC) than POC of the current picture, abackward STRP set comprising pictures having greater POC than the POC ofthe current picture, and a long-term reference picture (LTRP) set. 20.The non-transitory decoder-readable storage medium of claim 19, whereinthe reference picture list includes reference picture list 0 andreference picture list 1, wherein the reference picture list 0 isconstructed by sequentially allocating reference picture indices toforward STRPs forming the forward STRP set, backward STRPs forming thebackward STRP set and LTRPs forming the LTRP set; and wherein thereference picture list 1 is constructed by sequentially allocatingreference picture indices to the backward STRPs forming the backwardSTRP set, the forward STRPs forming the forward STRP set and the LTRPsforming the LTRP set when the prediction target block is a B slice. 21.The non-transitory decoder-readable storage medium of claim 16, whereinderiving the reference picture list comprises: deriving a temporaryreference picture list based on decoded pictures; and deriving thetemporary reference picture list as the reference picture list when thelists modification present flag indicates that the information onmodification of the reference picture lists is not present.